When drilling a borehole in an earth formation, for example to create a well for exploration or production of mineral hydrocarbons, a steel casing is commonly set at a certain stage in the drilling operation in order to secure an already drilled section of the bore hole before drilling deeper. After setting the casing, a drill bit is run through the casing in order to reach the bottom of the hole where the drilling operation can be continued. Running the drill bit through the casing involves a risk of damaging the casing wall, in particular since normally a fresh drill bit is selected that has sharp rock cutters, usually in the form of shear cutters which are much harder than the casing steel and are even capable of cutting through the casing. Since a casing is generally intended to remain in the borehole for a long period of time, it is undesirable to cause casing wall damage.
Moreover, a casing is often cemented in the borehole. Commonly applied cementing procedures require that some cement as well as auxiliary equipment such as cementing plugs remain inside the casing shoe. This must be drilled out in order to expose the bottom of the borehole for continued drilling. However, drilling out the remaining cement and auxiliary equipment from the casing involves the risk of damaging the casing with the rock cutters of the drill bit. This is particularly so if it is desired to leave as little as possible remains of cement in the casing, for example if the lower end portion of the casing is to receive the upper end portion of another casing or liner in sealing engagement.
Thus there is a need for an improved method of removing a body of selected material from the interior of a wellbore casing, which method overcomes the aforementioned problems.